The present invention relates to magnetoresistive read (MR) sensors for reading signals recorded in a magnetic storage medium, and more particularly, this invention relates to improving the design of an MR sensor.
Computer systems generally utilize auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an xe2x80x9cMR elementxe2x80x9d) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization of the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic storage medium because the external magnetic field from the recorded magnetic storage medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material separated by a layer of non-magnetic electrically conductive material are generally referred to as spin valve (SV) sensors manifesting the GMR effect (SV effect). In a spin valve sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO, FeMn, PtMn) layer. The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic storage medium (the signal field).
In spin valve sensors, the spin valve effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic storage medium because the external magnetic field from the magnetic storage medium causes a change in the direction of magnetization in the free layer, which in turn causes a change in resistance of the spin valve sensor and a corresponding change in the sensed current or voltage.
FIG. 1 shows a typical spin valve sensor 100 (not drawn to scale) comprising end regions 104 and 106 separated by a central region 102. The central region 102 has defined edges and the end regions are contiguous with and abut the edges of the central region. A free layer (free ferromagnetic layer) 110 is separated from a pinned layer (pinned ferromagnetic layer) 120 by a non-magnetic, electrically-conducting spacer 115. The magnetization of the pinned layer 120 is fixed through exchange coupling with an antiferromagnetic (AFM) layer 125. An underlayer 126 is positioned below the AFM layer 125.
The underlayer 126, or seed layer, is any layer deposited to modify the crystallographic texture or grain size of the subsequent layers, and may not be needed depending on the substrate. A variety of oxide and/or metal materials have been employed to construct underlayer 126 for improving the properties of spin valve sensors. Often, the underlayer 126 may be formed of tantalum (Ta), zirconium (Zr), hafnium (Hf), or yttrium (Y). Ideally, such layer comprises NiFeCr in order to further improve operational characteristics.
Free layer 110, spacer 115, pinned layer 120, the AFM layer 125, and the underlayer 126 are all formed in the central region 102. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed over hard bias layers 130 and 135, respectively, provide electrical connections for the flow of the sensing current Is from a current source 160 to the MR sensor 100. Sensor 170 is connected to leads 140 and 145 senses the change in the resistance due to changes induced in the free layer 110 by the external magnetic field (e.g., field generated by a data bit stored on a disk). IBM""s U.S. Pat. No. 5,206,590 granted to Dieny et al. and incorporated herein by reference, discloses an MR sensor operating on the basis of the spin valve effect.
Another type of spin valve sensor is an anti-parallel (AP)-pinned spin valve sensor. FIG. 2A shows an exemplary AP-Pinned spin valve sensor 200 (not drawn to scale). Spin valve sensor 200 has end regions 202 and 204 separated from each other by a central region 206. AP-pinned spin valve sensor 200 comprises a Nixe2x80x94Fe free layer 225 separated from a laminated AP-pinned layer 210 by a copper spacer layer 220. The magnetization of the laminated AP-pinned layer 210 is fixed by an AFM layer 208, or pinning layer, which is made of NiO. Again, beneath the AFM layer 208 is an underlayer 209.
The laminated AP-pinned layer 210 includes a first ferromagnetic layer 212 (PF1) of cobalt and a second ferromagnetic layer 216 (PF2) of cobalt separated from each other by a ruthenium (Ru) anti-parallel coupling layer 214. The AMF layer 208, AP-pinned layer 210, copper spacer 220, free layer 225 and a cap layer 230 are all formed sequentially in the central region 206. Hard bias layers 235 and 240, formed in end regions 202 and 204, provide longitudinal biasing for the free layer 225. Electrical leads 245 and 250 are also formed in end regions 202 and 204, respectively, to provide electrical current from a current source (not shown) to the spin valve sensor 200.
In use, the GMR effect depends on the angle between the magnetizations of the free and pinned layers. More specifically, the GMR effect is proportional to the cosine of the angle xcex2 between the magnetization vector of the pinned layer (MP) and the magnetization vector of the free layer (MF) (Note FIGS. 2B and 2C). In a spin valve sensor, the electron scattering and therefore the resistance is maximum when the magnetizations of the pinned and free layers are antiparallel, i.e., majority of the electrons are scattered as they try to cross the boundary between the MR layers. On the other hand, electron scattering and therefore the resistance is minimal when the magnetizations of the pinned and free layers are parallel; i.e., of electrons are not scattered as they try to cross the boundary between the MR layers.
In other words, there is a net change in resistance of a spin valve sensor between parallel and antiparallel magnetization orientations of the pinned and free layers. The GMR effect, i.e., the net change in resistance, exhibited by a typical prior art spin valve sensor is about 6% to 8%.
In order to effect the necessary pinning of at least one of the ferromagnetic layers to afford proper operation, it is necessary to include the hard bias layers in the context of the MR sensors of FIG. 1 and FIG. 2A. It should be noted that the pinning may also be achieved using antiferromagnetic (AFM) layers in lieu of the hard bias layers. In any case, such additional hard bias or AFM layers contribute to the size of the MR sensor. Unfortunately, this required component limits the ability of designers to reduce the overall size of MR sensors.
FIG. 2D illustrates a magnetic storage medium 280 from which the MR sensors of FIG. 1 and FIG. 2A are adapted to extract data. As shown, the magnetic storage medium 280 includes a plurality of transitions between a first magnetic state 282 and a second magnetic state 284. It is through these changes in state that data is stored and extracted.
Traditionally, prior art MR sensors such as those of FIG. 1 and FIG. 2A are only adapted to read one state at a time. Unfortunately, this technique does not take advantage of both states. For example, if one state (i.e. first magnetic state 282) which is being read is defective in some manner, the data being extracted may be erroneous.
There is thus a need for a MR sensor design and associated method of manufacturing the same which are capable of reducing the size of the MR sensor, and further read more than one state at a time.
A magnetoresistive read (MR) sensor system and a method for fabricating the same are provided. First provided are a first ferromagnetic layer, a first spacer layer positioned above the first ferromagnetic layer, and a second ferromagnetic layer positioned above the first spacer layer for working in conjunction with the first ferromagnetic layer to define a first sensor. An antiparallel coupling layer is positioned above the second ferromagnetic layer to separate the first sensor from a second sensor.
The second sensor is defined by a third ferromagnetic layer positioned above the antiparallel coupling layer, a second spacer layer positioned above the third ferromagnetic layer, and a fourth ferromagnetic layer positioned above the second spacer layer.
In one embodiment, the second ferromagnetic layer and the third ferromagnetic layer are self-pinned. Further, the first ferromagnetic layer and the fourth ferromagnetic layer may operate as free layers. For a Longitudinal recording system, the first ferromagnetic layer and the fourth ferromagnetic layer may be separated by xc2xd a bit length.
In another embodiment, the first sensor may be adapted to read a first magnetic state of a magnetic storage medium, and the second sensor may be adapted to simultaneously read a second magnetic state of the magnetic storage medium, where the magnetic storage medium includes a perpendicular recording system with the first and second magnetic states affording opposite polarity fields at the first and the second sensor.
In still another embodiment, the second ferromagnetic layer and the third ferromagnetic layer work in conjunction to serve in place of an antiferromagnetic layer. As an option, the antiparallel coupling layer may include Ru, the first and second spacer layers may include Cu, and the ferromagnetic layers may include CoFe.
To promote the self-pinned affect, the second ferromagnetic layer may have a first thickness that is different from a second thickness of the third ferromagnetic layer. Thus, no AFM layer or shields are necessarily required, and the present self-pinned MR sensor is ideally adapted for reading a perpendicularly recorded storage medium.